Liquid crystals have found widespread use since the first commercially usable liquid-crystalline compounds were found about 30 years ago. Known areas of application of conventional mixtures are, in particular, displays for watches and pocket calculators, and large display panels as used in railway stations, airports and sports arenas. Further areas of application are displays of portable and desktop computers, navigation systems and video applications. For the last-mentioned applications in particular, high demands are made of the response times and contrast of the images.
The spatial arrangement of the molecules in a liquid crystal has the effect that many of its properties are direction-dependent. Of particular importance for use in liquid-crystal displays are the optical, dielectric and elastomechanical anisotropies. Depending on whether the molecules are oriented with their longitudinal axes perpendicular or parallel to the two plates of a capacitor, the latter has a different capacitance; in other words, the di-electric constant ε of the liquid-crystalline medium has different values for the two orientations. Substances whose dielectric constant is larger when the longitudinal axes of the molecules are oriented perpendicular to the capacitor plates than when they are oriented parallel are referred to as dielectrically positive. In other words, if the dielectric constant ε∥ parallel to the longitudinal axes of the molecules is larger than the dielectric constant ε⊥ perpendicular to the longitudinal axes of the molecules, the dielectric anisotropy Δε=ε∥−ε⊥ is greater than zero. Most liquid crystals used in conventional displays fall into this group.
Both the polarisability of the molecule and the permanent dipole moment play a role for the dielectric anisotropy. On application of a voltage to the display, the longitudinal axis of the molecules orients itself in such a way that the larger of the dielectric constants becomes effective. The strength of the interaction with the electric field depends on the difference between the two constants.
In the case of the liquid-crystalline molecules used in conventional liquid-crystal displays, the dipole moment oriented along the longitudinal axis of the molecules is greater than the dipole moment oriented perpendicular to the longitudinal axis of the molecules.
By means of liquid crystals in which the greater dipole moment is oriented parallel to the longitudinal axis of the molecule, very high-performance displays have already been developed. In most cases here, mixtures of from 5 to 20 components are used in order to achieve a sufficiently broad temperature range of the mesophase and short response times and low threshold voltages. However, difficulties are still caused by the strong viewing angle dependence in liquid-crystal displays as are used, for example, for laptops. The best imaging quality can be achieved if the surface of the display is perpendicular to the viewing direction of the observer. If the display is tilted relative to the observation direction, the imaging quality deteriorates drastically under certain circumstances. For greater comfort, attempts are being made to maximise the angle through which the display can be tilted from the viewing direction of an observer without significantly reducing the imaging quality. Attempts have recently been made to improve the viewing-angle dependence using liquid-crystalline compounds whose dipole moment perpendicular to the longitudinal axis of the molecule is larger than that parallel to the longitudinal axis of the molecule. The dielectric anisotropy Δε is negative in this case. In the field-free state, these molecules are oriented with their longitudinal axis perpendicular to the glass surface of the display. Application of an electric field causes them to orient themselves more or less parallel to the glass surfaces. In this way, it has been possible to achieve an improvement in the viewing-angle dependence. Displays of this type are known as VA-TFT (“vertically aligned”) displays.
Development in the area of liquid-crystalline materials is still far from complete. In order to improve the properties of liquid-crystalline display elements, attempts are constantly being made to develop novel compounds which enable optimisation of such displays.
It is an object of the present invention to provide compounds having advantageous properties for use in liquid-crystalline media. In particular, they should have negative dielectric anisotropy, which makes them particularly suitable for use in liquid-crystalline media for VA displays. Irrespective of the dielectric anisotropy corresponding to the display type, compounds are desired which have a favourable combination of the applicational parameters. Of these parameters, which are to be optimised simultaneously, particular mention should be made of a high clearing point, a low rotational viscosity, an optical anisotropy in the use range, and the properties which serve to achieve mixtures having the desired liquid-crystalline phases over a broad temperature range (lower melting point, good miscibility with other liquid-crystalline components of the desired type).
Further LC display modes, which are also used, in particular, for small and medium-sized LC displays for use in portable devices, such as, for example, tablet PCs or so-called smartphones, are the IPS mode and the FFS (fringe field switching) mode, in which LC media having positive dielectric anisotropy are used. The prior art discloses that the properties of a liquid-crystal display of the FFS type can be improved by adding liquid-crystal materials having negative dielectric anisotropy to highly polar LC media having positive dielectric anisotropy, causing the dielectric constant ε⊥ perpendicular to the longitudinal molecular axes of the LC mixture to be increased (see EP 2 628 779 A2). Consequently, the high negative dielectric anisotropy of the admixed substances must be compensated again by a higher proportion of materials having positive dielectric anisotropy in order to produce the polarity of the mixture which is necessary for switching. There is therefore a need for LC mixture components which, although having a high ε∜, reduce the polarity of an LC mixture having positive Δε to a lesser extent owing to a relatively low Δε.
A further object of the present invention is therefore to provide compounds which, besides the above-mentioned advantageous applicational properties, have particularly high values of ε⊥ at the same time as relatively low values of Δε. In other words, the ratio of ε⊥ to |Δε| must be as large as possible.
Upon further study of the specification and appended claims, other objects, aspects and advantages of the invention will become apparent. The prior art discloses VA materials which are derived from dibenzofuran or from dibenzothiophene.
WO 02/055463 discloses compounds of the formula
in which X can denote, inter alia, O or S, Y can denote F, R1 and R2 can denote alkyl or alkoxy, and the other parameters have the meaning indicated therein. The compounds described therein have negative dielectric anisotropy, but were developed for ferroelectric LC mixtures and no values for the dielectric anisotropies of the individual substances are described.
Similar compounds are disclosed in DE 10 2004 021 691 A1, in which the group X generically includes polar radicals, such as F or —CF3, but preferably denotes H.

DE 10 2005 012 585 A1 describes, inter alia, dibenzofuran and dibenzothiophene compounds of the general formula
in which, inter alia, Y can denote O or S, the radical R1 can denote alkoxy, R2 can denote H, m and n can denote O and the radicals X1, X2 and X3 can denote F, as, for example, in the compound of Example No. 97

The compounds are highly polar and were developed as components for liquid-crystal mixtures for VA displays.